1. Field
Embodiments of the disclosure relate generally to the field of scanning probe microscopy and more particularly a head incorporated in a scanning probe microscope incorporating offset angles introduced between the beam axis and a reflecting surface normal on the cantilever.
2. Background
A Scanning Probe Microscope (SPM) scans a physical probe relative to an object in order to form an image. This may be achieved by movement of either the probe or the object. In an Atomic Force Microscope (AFM) and its many derivatives, the probe is attached to the end of a cantilever, which is also scanned relative to the object. The force exerted on the probe by interaction with the object, deflects the cantilever. Cantilever deflection is then measured by either optical or electrical methods. Electrical methods, such as piezo-electric detection, piezo-resistive detection, capacitive detection, and scanning tunneling microscopy are rarely used in practice due to sensitivity, complexity, and cost limitations. Optical methods measure either the amplitude or phase of light to determine cantilever deflection. Measuring the phase of light requires interference with a reference beam to transform the phase shift into an amplitude shift prior to photo-detection. Optical interferometry methods are also rarely used in practice due to sensitivity, complexity, and cost limitations. Detecting changes in the amplitude of light is the simplest and therefore most commonly used method for measuring cantilever deflection.
In standard cantilever SPM, light from a laser is reflected near the end of the cantilever and its amplitude is measured by a position sensitive detector. The planar reflection surface may be the cantilever itself or a mirror mounted on the cantilever. The cantilever mechanically transforms the magnitude of the force exerted on the probe into angular displacements of the reflecting surface. Reflection then transforms these angular displacements of the reflecting surface into angular displacements of the optical beam that are twice as large. Propagation away from the reflecting surface transforms these angular displacements into spatial displacements of the optical beam, which are then measured by the position sensitive detector.
Most standard cantilever SPM designs have a large head that includes the laser, detector, and a mechanical structure to attach them to the cantilever base. Vibration and drift in this mechanical structure create additional angular and spatial displacements, which limit sensitivity to the small angular displacements associated with probe forces. In object-scanning cantilever SPM, reducing the head size offers several advantages. The primary advantage is improved immunity to vibration and drift. Another advantage is the ability to meet the head size and mass budget associated with certain applications. For example, in SPM applications with an optical microscope, smaller dimensions permits the use of high numerical aperture objectives with short working distance, thereby improving spatial resolution and collection efficiency. In head-scanning cantilever SPM, reducing the head size offers additional advantages beyond those already described. One additional advantage is that the resonance frequencies and associated head scan rates can be significantly increased. Another additional advantage is that the size of the piezo and motor drive elements and their associate power requirements can be reduced.
The optical path in a cantilever SPM can be divided into two or more independent subsystems by guiding light through single-mode optical fiber. Removing the laser and detector from the head and placing them at the tail end of the fiber allows the dimensions of the head to be significantly reduced and eliminates the internal sources of thermal drift. A single-mode optical fiber can function as a bidirectional waveguide to both deliver the laser light and collect it, so only one fiber is necessary. Light propagating in the fundamental mode of standard single-mode fibers has an electromagnetic field distribution, which can be approximated as a Gaussian amplitude function with planar phase fronts. When launched out of a fiber facet, the field distribution may then be approximately described by Gaussian beam equations, where the beam waist is at the fiber facet. The beam radius is the radius from the optical axis where the optical intensity decreases by a factor of the mathematical constant e squared. The minimum value of the beam radius is at the waist, where it is half of the fiber Mode Field Diameter (MFD). The Rayleigh range is the distance from the beam waist position, along the optical axis, where the optical intensity drops to half of its peak value at the waist.
Single-mode optical fiber has been used in certain SPM head designs to reduce the head size down to just a single-mode optical fiber 102, a mechanical mount 104, and a probe cantilever 106 positioned relative to the single-mode optical fiber by the mount 104, as illustrated in FIG. 1. In these designs, changes in the phase of light, associated with longitudinal displacement of a reflecting surface on the probe cantilever are measured by interfering the returning light with reference light. The reflecting surface is designed to be normal to the beam axis, and therefore return as much light from the reflecting surface back into the fiber, as possible. To maintain reasonable efficiency, the reflecting surface diameter must be greater than the fiber MFD, and the distance between the fiber facet and reflection surface should be significantly less than the Rayleigh range associated with the fiber MFD. In such cantilever translation SPM head designs the fiber mode only acts as a waveguide for the incident and reflected light.
It is therefore desirable to provide scanning probe microscope head, which permits greater working distance between the fiber facet and the cantilever reflection surface, and modulates the amplitude of returning light with angular displacement.